To provide deflection or scanning of electron beams produced by cathode ray tubes, in particular television kinescopes, a deflection yoke produces electromagnetic fields in the vicinity of the beams. The yoke normally comprises a set of horizontal deflection coils and a set of vertical deflection coils. The horizontal and vertical field intensities are varied by varying deflection coil current in order to deflect the beams at the line and field rate, respectively, in response to deflection signals produced by deflection circuits.
In a conventional television display system, the deflection yoke is located on the outside of the kinescope in a region encompassing a portion of the cylindrical neck and flared funnel. In order to generate sufficient deflection flux density inside the kinescope in the vicinity of the beams, the deflection coils must contain relatively large amounts of wire, resulting in a heavy and expensive device. A yoke of this type is also wasteful of energy, as the deflection fields are not localized in the vicinity of the electron beams. Significant stray or leakage flux is also produced by the yoke, which, in addition to not contributing to the desired deflection of the electron beams, also may interact with the beams in the region of the electron gun assembly. This may cause beam defocusing and, in the case of a color television receiver, misconvergence of the beams.
Placing the deflection yoke inside the glass envelope of the cathode ray tube permits a reduction in deflection power with increased deflection sensitivity since the deflection fields may be localized close to the beams. The yoke may be made quite small, resulting in significant materials and cost savings.
Locating the deflection yoke inside the tube is subject to some problems, however. The insulation of conventional enameled deflection coil wire will not withstand the high tube baking temperatures (e.g., 400.degree. C.) encountered in the tube manufacturing process. These temperatures cause the enameling to be burned, resulting in the occurrence of short circuits in conventionally-wound deflection coils. The burned insulation also contaminates the tube internal environment. Also, the yoke, at approximately ground potential, may be located close to portions of the electron gun assembly which are at high potentials (e.g., 25 kv.). Arcing between internal tube components may occur.
As previously described, maximum deflection sensitivity and minimum deflection power consumption is achieved by placing the deflection coils as close as possible to the electron beams. The deflection coils, however, must have enough ampere-turns to develop sufficient flux density to properly deflect the beams. This requirement may result in an elongated yoke which has a relatively long deflection region. Since the deflection coils are close to the beams, beam deflection through the deflection region must be gradual to prevent the deflected beams from striking portions of the yoke. This results in narrow beam deflection angles, which is undesirable in television kinescopes, since it requires longer tubes. Television kinescopes preferably have large deflection angles (e.g., 90.degree.-110.degree.) resulting in smaller, more compact receivers.